The quadrupole mass spectrometers are well known in the art and find themselves applied in a variety of fields wherein ions are analyzed according to their m/e values, m being the mass of an ion and e its electrical charge.
As shown in U.S. Pat. Nos. 3,334,225, Aug. 1, 1967 (Langmuir), 3,413,463, Nov. 26, 1968 (Brubaker) and 4,214,160, July 22, 1980 (Fies et al), quadrupole mass spectrometers are normally operated using combined radiofrequency (RF) and continuous (DC) voltages applied to the rod electrodes. In this moade of operation, V.sub.RF, voltage of the RF, and V.sub.DC, voltage of the DC, are set in such a way that the mass spectrometer operates in the stability region (the first region of stability) near the origin depicted in the well known (a,q) diagram. Problems arise under these conditions in achieving (a) good transmission at high mass, (b) good resolution in a structure which can be cheaply manufactured and (c) consistently good peak shape. To avoid some of those problems, an RF-only quadrupole mass spectrometer was first described in U.S. Pat. No. 4,090.075, May 16, 1978 (Brinkman). Further improvements have been patented in the U.S. Pat. No. 4,189,640 Feb. 19, 1980 (Dawson) and British Pat. No. 1,539,607, Jan. 31, 1979 (Leck). In the RF-only quadrupole mass spectrometers, steps in the ion transmission versus voltage amplitude curves occur as each type of ion passes beyong the stability boundary. In the patent to Brinkman, step signals are converted to mass peak signals by the use of retarding electrodes or a mass analyzer at the output end of the quadrupole electrodes. The patent to Leck, on the other hadn, uses an annular detector for desired ions and a central electrode surrounded by the annular detector for unwanted ions. Dawson employs a centrally located "stop" to eliminate ions of higher mass with stable trajectories which generate background and associated noise. In their article in Dynamic Mass Spectrometry No. 5 (1978) pages 41-54, Chapter 2 "Modulation Techniques Applied to Quadrupole Mass Spectrometer", Weaver and Mathers report the use of modulation of the RF voltage amplitude to differentiate signals for converting the steps to mass peaks. Although the RF-only quadrupole mass spectrometers have proven very successful, this technique of Weaver and Mathers did not find application because noise on large transmitted signals prevented the detection of small signals, i.e. limited synamic range.